1. Field of the Invention
This invention relates to information storage systems using a disc format. In a primary application this invention relates to storing information on a disc which can be removed and then replaced for further information storage. Another application of this invention is the recording of new tracks on a previously recorded disc with the new tracks being uniformly spaced from the previously recorded tracks.
2. Description of Prior Art
Many commercially available disc recording systems are presently in use which utilize either optical, magnetic or mechanical techniques. In these systems there is usually some problem in achieving uniformly spaced tracks. This is particularly difficult if the information capacity on the disc is efficiently used and the tracks are closely packed. There is the danger of newly written tracks obliterating the previous tracks by crossing them.
In some applications, the entire disc is recorded during a single sequence without removing it and replacing it for further recorded information. This is the case, for example, in the MCA Disco-Vision System where the entire disc is recorded at one time and copies are made from it. In document storage systems, however, where individual documents are added at different times, the disc presents serious tracking problems. If the disc is removed and then remounted, the resultant eccentricities of the tracks from a slightly off-center mounting can cause the tracks to cross and thus be destroyed. If large guard spaces are used to avoid track crossing, the storage area is used inefficiently and a limited number of documents can be stored.
One solution to this problem is described in the previously referenced U.S. patent application Ser. No. 781,056. Here an optical tracking system is used to follow a previously recorded track and control the radial position of the newly recorded tracks so that they will be uniformly spaced from the previous tracks. This system, however, is relatively expensive to implement in that it requires an additional laser light source for the tracking operations while recording is taking place. In addition, it is limited to those systems where the newly written tracks are immediately available to be read for tracking purposes. Thus, for example, recording on photographic materials, which requires subsequent development, could not be used with this tracking system.